Filtration of fluids may be accomplished through a variety of technologies, the selection of which is often determined by the contaminant or contaminants that are being targeted for removal or reduction. Particulates are best removed through a process known as depth filtration. The filter collects and holds any dirt or sediment within its matrix. Dissolved organic contaminants appearing on a molecular level may be removed through adsorption or, in the case of minerals and metals, through ion exchange. Very small contaminants, including microorganisms down to sub-micron sizes often require some form of membrane technology in which the pores in the membrane are configured to be smaller than the target contaminant; or they can be deactivated in some manner. Contaminants in drinking water may be broken down into four groups: (i) turbidity and particulates; (ii) organic based chemicals and pesticides; (iii) inorganic matter such as dissolved heavy metals that pose a health risk such as lead; and minerals; (iv) microorganisms such as protozoan parasites, bacteria and viruses.
Each group can be treated with different, specific technologies. A traditional approach to filtration that accommodates multiple types of contaminant groups is to arrange multiple discrete media modules in a linear fashion in separate housings along a path, each module sized in a way to protect sequential stages to the final stage for roughly optimal life of the system. Some filters, however, are designed to treat several contaminant groups through a single filtering technology. The single filtering technology is helpful for applications where highly efficient and compact geometries are warranted, such as in a point-of-use filter for consumers.
For example, serial filtration within a single housing can be achieved wrapping a block filter with single or multiple layers of wrapped or pleated non-wovens or membranes (e.g. U.S. Pat. Appl. Publ. No. 2004/0206882 to Hamlin et al.). These layers are applied to essentially monolithic blocks, with mixed functionality incorporated within the block as a blended monolithic structure. In the case of mixed functionality where a mixture of different active media are employed in the blended monolithic structure, the resultant block has a substantially isotropic pore size distribution, void volume, and geometry—dependent water flow pressure drop for the full depth of the bed. Formulations of these mixed active media blocks are designed with an appropriate trade off between the functionality of one component versus the other, and the cost of the media is affected by the need to select particle size distributions and other media characteristics that result in the best blended performance. As an example, a low cost large particle of activated carbon media (resulting in relatively larger pores in a formed article block) may be sufficient for a chlorine reduction claim. A smaller, higher cost active media particle (resulting in relatively smaller pores in a formed article block) may be required for a cyst claim. The blend of large particles and small particles may create an isotropic pore size distribution that is wider than the individual distributions of either individual component blocks, and may thus be too wide to efficiently retain the cyst. As such, the blended monolithic block may not be as effective in performance for the combination of claims as individual functionality. The engineering trade-off normally employed is to select materials and blend formulations that meet the critical or more difficult claim first, which requires either cost compromise in material, or pressure-drop compromise in the finished monolithic block.
Additionally, the limitations of a mixture of active media include the effect of dilution of mechanism in a mixed system; not all filtration and separation mechanisms are synergistic in their effect. With respect to electrokinetic capture, it is generally believed that a smaller volume having a greater population/concentration of active media functionality can be more effective in acting on a fluid stream than a more dispersed and diluted population of the same particles; especially if the effect of this dilution is to change the geometric relationship between the fluid path and the surface of the particle (by, for example, creating effectively larger pores).
In addition to the aforementioned trade-offs, most commercial blended monolithic blocks are produced using binder particles having undesirable characteristics of melt-flow and coating of active media in neighboring active media particles; this coating may generally result in blinding off some percentage of the surface of the active media.
There is an ongoing need to provide filtration media and systems for highly efficient and compact applications. There also exists a need with regard to depth filter media such as blocks, for a mechanism for reduction of phage, virus, or bacteria that is not dependent on the pore size or pore size distribution of the block; especially where the block pore characteristics cannot effectively reduce a large microorganism such as a cyst.